The invention pertains to liquid crystalline side-chain polyesters prepared by polycondensing a dicarboxylic acid or a derivative thereof with diols of a certain general formula. Such diols display a molecular hyperpolarizability .beta. of the order of 10.sup.-38 m.sup.4 /V. The phenomenom of molecular hyperpolarizability and the related non-linear optical (NLO) effects (Pockel's effect and second harmonic generation) are described in ACS Symposium Series 233, American Chemical Society, Washington, D.C., 1983, and in Angew, Chem. 96 637-51 (1984).
Generally, hyperpolarizable molecules have a delocalized .pi. electron system to which both an electron donor group and an electron acceptor group are coupled directly (D.pi.A system).
Polymers having hyperpolarizable side groups may be subjected to polar orientation in an electric field. As a result, the material also becomes macroscopically hyperpolarizable.
Such material may be used in an optical switch, in which case an electric circuit is provided on a hyperpolarizable polymer. Such a use is described by J. J. Thackera et al., Appl. Phys. Lett. 52 1031-33 (1988).
The invention relates to liquid crystalline side-chain polyesters obtained by polycondensing a dicarboxylic acid or a derivative thereof with a diol of the formula: ##STR1## wherein R.sub.1 =halogen, R.sub.2, OR.sub.2 , COR.sub.2 , COOR.sub.2, CN[B] or CF.sub.3 ;
R.sub.2 =H or an alkyl group having 1-3 carbon atoms; PA1 Y=CR.sub.2, C--CN, or N; PA1 A=CN, NO.sub.2, CH=C(CN).sub.2, ##STR2## or CF.sub.3 ; n=0-4; and PA1 x=0-6. PA1 R.sub.2 H or an alkyl group having 1-3 carbon atoms; PA1 Y is CR.sub.2, C--CN or N; PA1 A is CN, NO.sub.2, CH.dbd.C(CN).sub.2, ##STR4## or CF.sub.3 ; n is 0-4; and PA1 x is 0-6.
The groups R.sub.1 at the two benzene rings are not necessarily identical. This also applies to the groups R.sub.2 at the double bonds.
The essence of the polyesters according to the invention is that they are prepared from diols having a D.pi.A system. The group at the benzene ring (R.sub.1) is not of vital importance here. Only if there should be a fluorine substituent at the ring to which the acceptor A is also coupled may, on the strength of European Patent Specification No. 0 241 338, a real effect be expected on the macroscopic hyperpolarizability following the incorporation of the diol in a polymer. When selecting substituents at the double bond it should be kept in mind that the D.pi.A system must not be forced out of planarity. If it is sterically forced out of planarity slightly by, for instance, a t-butyl group, then the hyperpolarizability may be influenced negatively.
Further, the polyesters according to the invention are particularly suitable as a medium for optical wave guides and for optical data storage. In general, the use of liquid crystalline side-chain polymers for optical data storage is known from DE No. 3603267, which disclosure is incorporated herein by reference. In this respect the values of Tg (glass transition temperature) and Tc (clearing temperature) are of importance.
Tc is a concept known to the artisan and is used to characterize the thermodynamic stability of a liquid crystalline phase. It marks the transition from the anisotropic to the isotropic liquid phase.